The purpose of this study is to identify changes in resting energy expenditure (REE), food intake, growth velocity, body composition, and pulmonary function over six months in patients prescribed DNAse (Pulmozyme) for treatment of the pulmonary disease of cystic fibrosis (CF). Growth failure and malnutrition are common clinical features in CF. Previous studies have implicated various components of energy imbalance as the cause of malnutrition and suggested that improved nutritional status can slow the progression of pulmonary disease and increase life expectancy. We hypothesize that DNAse therapy will be associated with a decrease in the elevated REE seen in children with CF, with improved linear growth and body weight velocity, and with improved nutritional status as indicated by increased total body fat and subcutaneous fat stores after six months of treatment. We are currently completing enrollment for this study. To date 22 subjects have completed baseline evaluations, 15 subjects have completed the 2 month evaluation, and one subject has completed the final 6 month evaluation. Three new subjects have been identified and scheduled to begin within the next few weeks, and five follow-up evaluations have been scheduled. Two subjects have withdrawn from the study after baseline due to financial constraints involved in purchasing the DNAse. Overall, the study is proceeding well and no adverse circumstances have been encountered. We expect to complete recruitment within the next 6 weeks.